An anesthesia machine is a medical device that delivers a controlled mixture of gases and anesthetic vapor to keep a patient unconscious, pain free, and safely oxygenated during surgery. It combines oxygen, medical air, sometimes nitrous oxide, and volatile anesthetic agents in carefully measured amounts. The machine also supports breathing when a patient cannot breathe normally on their own.
This matters because small errors in gas delivery, ventilation, or monitoring can quickly affect the brain, heart, and other organs.
Key Facts
- Fresh gas flow is the total flow of oxygen, air, nitrous oxide, and anesthetic vapor delivered to the breathing circuit.
- FiO2 = oxygen flow / total fresh gas flow when only oxygen and non-oxygen gases are mixed.
- Minute ventilation = tidal volume x respiratory rate.
- A vaporizer adds a controlled concentration of liquid anesthetic vapor to the carrier gas stream.
- The CO2 absorber removes exhaled carbon dioxide so rebreathed gas does not cause CO2 buildup.
- Pulse oximetry estimates blood oxygen saturation, while capnography measures exhaled CO2.
Vocabulary
- Anesthesia machine
- A device that mixes medical gases and anesthetic vapor, delivers them to a patient, supports ventilation, and helps monitor breathing.
- Vaporizer
- A calibrated component that turns liquid anesthetic into vapor and adds a set concentration to the gas flow.
- Breathing circuit
- The tubing and valves that carry gas from the machine to the patient and return exhaled gas for removal or reuse.
- Ventilator
- A mechanical system that moves gas into and out of the lungs when a patient needs breathing support.
- Capnography
- The continuous measurement and display of carbon dioxide in exhaled breath.
Common Mistakes to Avoid
- Confusing oxygen flow with total fresh gas flow. Oxygen flow is only one part of the mixture, so FiO2 depends on all gas flows being delivered.
- Ignoring the CO2 absorber. A used or missing absorber can allow the patient to rebreathe carbon dioxide, which can cause dangerous CO2 buildup.
- Assuming the vaporizer output equals the amount reaching the lungs. Leaks, circuit settings, uptake by the patient, and fresh gas flow can affect delivered concentration.
- Treating pulse oximetry as a ventilation monitor. Oxygen saturation can stay normal for a short time even when ventilation is poor, so capnography is needed to assess CO2 removal.
Practice Questions
- 1 An anesthesia machine delivers 2 L/min of oxygen and 3 L/min of medical air. Assuming medical air is 21 percent oxygen, what is the approximate FiO2 of the total fresh gas flow?
- 2 A ventilator is set to a tidal volume of 500 mL and a respiratory rate of 12 breaths/min. What is the minute ventilation in L/min?
- 3 A patient has a normal pulse oximeter reading but the capnography waveform disappears during surgery. Explain what this could indicate and why the anesthesia team must respond quickly.